A base material used for the laminated materials for electrical circuit boards must have excellent properties such as heat resistance, heat-resistant dimensional stability, humidity-resistant dimensional stability, electrical insulation and resistance against deformation (hardly twisting, warping or undulating). In the electrical circuit boards for small electronic devices such as cell phones, notebook personal computers and the like, further, the wiring must be formed very densely. Therefore, the base material therefor must meet various properties, such as low weight, in addition to the above-mentioned properties. The heat-resistant synthetic fiber sheet features excellent heat resistance, heat-resistant dimensional stability and light weight, as compared to sheet members made of other materials and has, in recent years, been used as a base material for the laminated material for electrical circuit boards that require the various properties described above.
For example, there have been known an electrically insulating aromatic polyamide fiber sheet comprising polymetaphenylene isophthalamide staple fibers (trademark: Conex, manufactured by Teijin Co.) and a polymetaphenylene isophthalamide pulp (fibrids) (Japanese Unexamined Patent Publications (Kokai) Nos. 2-236907 and 2-10684); a resin-impregnated aromatic polyamide fiber sheet comprising polyparaphenylene terephthalamide staple fibers (trademark: Kevlar, manufactured by du Pont Co.) or a copolyparaphenylene-3,4′-oxydiphenylene terephthalamide staple fibers (trademark: Technora, manufactured by Teijin Co.) and an organic resin binder (Japanese Unexamined Patent Publication (Kokai) No. 1-92233); and a method of producing the above aromatic polyamide fiber sheets (Japanese Unexamined Patent Publication (Kokai) No. 2-47392).
The former electrically insulating aromatic polyamide fiber sheet has excellent heat resistance. When heat-treated at a temperature of not lower than 250° C., however, this electrical insulating aromatic polyamide fiber sheet shrinks and changes in size. Besides, the fiber has a high equilibrium moisture content (water content) as well as a high impurity ion content. When placed for extended periods of time under highly humid condition, therefore, the electrical insulation becomes insufficient. Therefore, this aromatic polyamide fiber sheet cannot be used as a base material for electrical insulation where a high degree of reliability is required.
On the other hand, the latter resin-impregnated aromatic polyamide fiber sheet has a small equilibrium moisture content and a relatively small impurity ion content, but uses the organic resin only as a binder component. During the step of producing the aromatic polyamide fiber sheet, therefore, the binder component migrates to the front and back surface sides of the sheet and stays on the front and back surface sides. As a result, the amount of the binder component becomes small in the middle layer of the sheet; i.e., the resin distribution in the resin-impregnated aromatic polyamide fiber sheet loses uniformity in the direction of thickness, and performance becomes less reliable.
When the above-mentioned conventional heat-resistant synthetic fiber sheet is used as a base material for electrical insulating materials, therefore, dispersion increases in the amount of the blended varnish that is impregnated (particularly, in the direction of thickness) or that is adhered in the step of production, particularly, during the step of preparing a prepreg by impregnating and drying a blended varnish, such as an epoxy resin, and in the step of laminating the obtained prepreg articles. Besides, the binder resin partly melts, whereby the adhering force among the fibers decreases and the sheet base material is caused to be broken. Further, the staple fibers move easily relative to each other, and the distribution of fiber densities loses uniformity causing the laminated material for electrical circuit boards to be deformed, particularly after the step of solder reflow that is carried out at a high temperature.
In order to solve the above problems inherent in the prior art, Japanese Unexamined Patent Publication (Kokai) No. 2001-295191 discloses a base material for the laminated material for electrical circuit boards, free of the above problems, obtained by using a fiber sheet that contains, as constituent elements, para-aromatic polyamide staple fibers, having two or more annular projections formed independently from each other in the longitudinal direction, and organic high molecular polymer fibrids.
In the para-aromatic polyamide staple fibers for the base material, however, two or more annular projections are, in many cases, formed on both flat end faces or on the portions close to both flat end faces of the staple fibers. Such projections promote the entanglement among the staple fibers and, hence, the fibers are not fully opened in the step of paper-making. In order to obtain a synthetic fiber sheet in which the staple fibers are uniformly distributed in the direction of plane and in the direction of thickness, therefore, a step must be provided to disaggregate the staple fibers to a sufficient degree causing, however, a drop in the productivity.
Further, if the synthetic fiber sheet is prepared in a state where the fibers are not fully opened, non-uniform portions are contained in the laminated material for electrical circuitry that is obtained. In particular, after the step of solder reflow executed at a high temperature, the laminated material for electrical circuit boards undergoes the problem of deformation. It has, therefore, been desired to solve this problem.